Steel is the generic name for a group of ferrous metals, composed principally of iron, which have considerable durability and versatility. By the proper choice of carbon content, addition of alloying elements, and by suitable heat treatment, different kinds of steel can be made for various purposes and the use in industry of all kinds of steel is now quite expansive.
Stainless steel (SS) is defined as a steel alloy, with a minimum of 11% chromium content by mass. Stainless steel does not stain, corrode, or rust as easily as traditional steel. There are over 150 different grades and surface finishes to allow the stainless steel to suit the environment in which it will be used. Stainless steel's low maintenance and relatively low cost make it an ideal base material for many commercial applications. It is used in cookware, cutlery, hardware, surgical instruments, major appliances, industrial equipment, food and beverage processing industry equipment. It is also used as a structural alloy for cars and as a construction material for buildings.
Stainless steels have a passive film of chromium oxide that forms in the presence of oxygen due to the chromium present in the steel. This layer blocks most corrosion from spreading into the metal's internal structure. High corrosion resistance can be achieved with chromium additions of 13% by weight up to 26% for harsh environments. The chromium forms a passive layer of chromium III oxide (Cr2O3) when exposed to oxygen. To have their optimum corrosion resistance, stainless steel surfaces must be clean and have an adequate supply of oxygen to maintain this passive surface layer.
Cleaning of stainless steel includes the removal of various surface contaminants to ensure corrosion resistance, to prevent contamination, and to achieve the desired appearance of the steel. Acid cleaning is a process by which a solution of a mineral and/or organic acid in water sometimes in combination with a wetting agent or detergent or both, is employed to remove iron and other metallic contamination, light oxide films, soil and similar contaminants.
Acid cleaning compositions for removing contaminants from stainless steel generally have the mineral or organic acid in a solution with a pH of less than 7.0. The compositions can remove both organic and inorganic soils in the same operation. They also are used to improve corrosion resistance and enhance brightness or gloss of the base metal surface.
One of the problems which arise in the use of steel is its corrosion, either by the atmosphere or by the environment in which it is used. The rate of corrosion may vary, depending on the surrounding conditions and also the composition of the steel. Stainless steel, especially, is much more resistant to corrosion as compared to carbon steels and other steels. The corrosion resistance of stainless steel is due to the addition of chromium and other metals to this alloy. Although stainless steel has appreciable resistance to corrosion, it will still corrode in certain circumstances and attempts have been made to prevent or reduce this corrosion. Most acid cleaners also include a corrosion inhibitor of some sort. For example, in acid media copper sulfate has been used as a corrosion inhibitor. However this and other proposed inhibitors are not entirely satisfactory since, like copper sulfate, they may be expensive, introduce an effluent disposal problem and, moreover, are not entirely effective. For example, when copper containing urea sulfuric solutions are placed in contact with nickel metal, copper will plate the nickel surface.
A variety of compounds, including dialkylthioureas, such as diethylthiourea and dibutylthiourea, are known to reduce the corrosivity of sulfuric acid to carbon steels. Thioureas are not appropriate for food and beverage situations as some of them have been found to pose potential health risks and any remnant thioureas compounds are considered contamination for such surfaces.
The type of acid used has also presented problems in development of acid cleaners. Many acid cleaners are based upon phosphoric acid due to its diverse functionality such as a low corrosion profile on many alloys and elastomers, good mineral solubility and good soil suspension properties. Many acid cleaners are also based on high levels of nitric acid due to its compatibility with a variety of materials as well as its effectiveness at mineral soil solubility and removal. However, high nitric acid based cleaners can cause vapor staining and corrosion to stainless steel due to the volatile airborne nitrogen oxides.
Phosphoric acid and nitric acid continue to have more strict effluent regulations due to the phosphorus and nitrate environmental and drinking water issues. It is therefore an object of this invention to provide a phosphorus free and reduced nitric acid based cleaning composition which has equal or superior cleaning, corrosion and vapor stain inhibiting properties as other phosphoric and nitric acid based cleaners on some varieties of stainless steel, such as the 300 series.
It is another object of this invention to provide aqueous, sulfuric based acid cleaning compositions which are relatively noncorrosive to stainless steel and which have a reduced cost.
Other objects, aspects and advantages of this invention will be apparent to one skilled in the art in view of the following disclosure, the drawings, and the appended claims.